In an integrated circuit, easily flexible thin substrates are used. As many materials such as metals, dielectric materials, and composites are used in the substrate, warpage occurs due to the difference in values of the coefficients of thermal expansion (CTE) of respective materials. The warpage imposes pressure on parts of an electric assembly, thereby producing problems. In order to prevent the warpage, a silica filler having a low coefficient of thermal expansion (CTE) is used as an underfill composition. A large amount of silica is introduced into the substrate in order to reduce the CTE, a silica additive for a circuit material is generally used by removing residual silanol groups on the surface of silica and being surface-treated with epoxy silane which is resin curable, and the quality of the circuit substrate may be improved by removing silanol groups on the surface of the silica filler through silica surface modification. The silica whose surface is modified with the epoxy group is cured with a compounded resin which is an underfill composition to improve mechanical properties of the substrate. Before the underfill composition is cured, a fully-blended varnish, which is a mixed solution, is prepared by dispersing a silica filler in a polar organic solvent, and then mixing the resulting dispersion with a compounded resin exhibiting hydrophobicity. Various resins, pigments, and curing agents are included in the compounded resin. It is judged that among resins used in the preparation of varnish, a cyanate resin causes the biggest problem in compatibility due to the difference in polarity with a silica filler, and when residual silanol groups are not sufficiently blocked because the surface treatment of the silica filler is not uniform, or the compatibility between an epoxy group which is a silica surface treatment agent and a cyanate resin is low, the viscosity of varnish which is a mixed solution is significantly increased to reduce the process efficiency prior to the curing of the composition, and problems are caused to the process of producing a stable product.
As an invention in the related art, in Korean Patent Application Laid-Open No. 10-2009-0090324, Japanese Patent No. 4276423, and the like, studies have been conducted on s silica filler for a circuit material by blocking silanol groups on the surface of the silica filler, but residual silanol groups on the surface of the silica filler are not sufficiently blocked by the related art and, there still remains a problem in that the viscosity of the composition is significantly increased.